There are times when advertising an hourly rate may be useful -
just starting out and want to drum up some business? - advertise a special 'new customer' rate....or a discount rate for senior citizens... or an 'as low as $' rate...like the Doc said, "After the phone rings is when the 'sales' work really starts." Your advertised 'as low as' rate might not be applicable to that particular inquiry, etc.
In '92 I advertised a low hourly rate special...got me a 30-day (8 hrs. per day) contract during a slow time...I still do work there at my normal rates.